Because of increase of telecommunication needs, an optical line such as a PON (Passive Optical Network) is spread as a subscriber telecommunication line instead of a metal line such as an ADSL (Asymmetric Digital Subscriber Line). The PON is an access-based optical network in which an optical terminal station device and a plurality of optical terminal devices are coupled with each other via an optical coupler (star coupler) branching a light, as disclosed in Japanese Patent Application Publication No. 2009-246537 (hereinafter referred to as Document 1).
The optical terminal station device is called an OLT (Optical Line Termination) or the like and is a transmission device provided in a station of an optical telecommunication service provider. The optical terminal device is called an ONU (Optical Network Unit) or an ONT (Optical Network Termination) or the like and is a transmission device provided in a subscriber of the optical telecommunication service. A transmission direction from the ONU to the OLT is defined as an upbound direction. A transmission direction from the OLT to ONU is defined as a downbound direction.
The optical coupler couples a single OLT with 32 ONUs, for example. Thus, the optical coupler achieves an optical network structure having a star type topology. One of advantages of the PON is to reduce the number of optical fibers (transmission lines) provided between a station and subscriber houses with use of the star type topology.
The OLT and ONU transmit a signal with a transmission speed of 1.25 Gbps or the like in the upbound direction and the downbound direction. The PON having the transmission speed of 1.25 Gbps is regulated as a G (Gigabit-capable)-PON in ITU-T (International Telecommunication Union Telecommunication Standardization Sector) recommendation G984 series. On the other hand, the PON having the same transmission speed based on an Ethernet technology is regulated as 1000 Base-PX20 (PX10) in IEEE (the Institute of Electrical and Electronics Engineers, Inc.) 802.3ah. The PON of 1000 BASE-PX20 (PX10) is called a GE (Gigabit Ethernet)-PON or the like.
Recently, further increase of the telecommunication needs is estimated. Therefore, the PON technology that has an upbound direction transmission speed and a downbound direction transmission speed of 2.5 Gbps and 10 Gbps and achieves a large capacity transmission is being developed. The technology is regulated as an XG (10-Gigabit-capable)-PON in ITU-T recommendation G987 series. On the other hand, the 10GE (10-Gigabit Ethernet)-PON having an upbound direction of 2.5 Gbps or 10 Gbps and a downbound direction of 10 Gbps is regulated in IEEE 802.3ay.
A downbound signal from the OLT to each ONU is processed for transmission by a TDM (Time Divisional Multiplexing) method as a continuous signal in which data for each ONU are continuously arranged. On the other hand, an upbound signal from each ONU to the OLT is processed for transmission based on an individual transmission timing instructed from the OLT to each ONU in advance so that each signal does not collide because the optical coupler and the OLT is coupled with a single optical fiber.
A guard time (no-signal period) that has a predetermined length and is determined based on on-off time of a transmission laser is provided between upbound signals adjacent to each other in order to suppress collision of the upbound signals. Therefore, the upbound signal is processed for transmission by a TDMA (Time Division Multiple Access) as a burst optical signal.
The OLT has an optical receiving device (burst receiving device) that receives a burst optical signal transmitted from each ONU. Amplifiers in the optical receiving devices are AC (Alternative Current) coupled with each other. That is, the amplifiers are coupled by a coupling capacitor. Or, the optical receiving device is coupled with a rear re-timing circuit by the AC coupling. When the AC coupling is used, it is not necessary to adjust output voltage levels of a positive phase electrical signal and a negative phase electrical signal obtained by conversion of the burst optical signal to an input level of a device to which the electrical signals are output, being different from a DC coupling. Therefore, a general electric device can be used, and a cost can be reduced.
Regarding an optical receiving device, for example, Document 1 discloses a technology that a section between bursts of a burst signal is detected, and an offset compensation is automatically performed. Japanese Patent Application Publication No. 2009-38556 discloses a technology that a DC offset voltage of a differential output signal of a differential amplification circuit is adjusted.